Pigments are widely used as colorants, for example, in paints, varnishes, and inks. Such pigments generally have average particle sizes (diameters) in the range of 0.1 to 10 micrometers, more typically, 1 micrometer or greater. To achieve these particle sizes, mechanical devices are most often used to comminute solid particulate into smaller primary particles. The most common mechanical devices include ball mills, attritors, sand/bead mills, and roll mills. All of these devices require moving parts in order to generate the mechanical forces required to break up the pigment particles. Milling times of several hours are typical, with certain pigments requiring a day or longer in order to break up, or comminute, the particles. Moreover, comminution of the pigment by contact with the milling media results in pigment surfaces exhibiting a high number of surface asperities. Furthermore, contamination of the dispersions from the mechanical parts of the milling equipment can result due to the intimate contact of the pigment with the milling media. Silicon dioxide, a grinding medium, is a common contaminant found after sand milling, for example.
Another inherent disadvantage of mechanical processing of pigments is the large breadth of distribution of particle sizes resulting from mechanical processes. This results in the presence of particles having diameters of one micrometer or greater, even in dispersions where the average particle size is significantly less. For dispersions requiring transparency in the final article, these larger particles lead to unwanted light scattering and are detrimental. The presence of these micrometer sized particles also leads to an inherent instability, or tendency to flocculate, in the dispersions. For a more general description of present limitations in dispersing solids in liquids, see D. J. Walbridge, Solid/liquid Dispersions, Th.F. Tadros, Ed., Academic Press, 1987, p. 50.
More stable pigment dispersions can be obtained by chemically altering the pigment as described in EP 1,544,839. This often results in smaller average particle diameters but has the disadvantages of requiring a chemical pretreatment of the pigment, still requiring mechanical milling, and still providing a dispersion having a wide particle size distribution.
Dry organic pigments, as for example members of the phthalocyanine family, have been generated by evaporative techniques. Wagner et al., J. Matls. Sci., 17, 2781 (1982), describes a train sublimator for purification of pigments in the phthalocyanine family with the main purpose of removing impurities present in the as-supplied pigments so as to allow preparation of ultrapure pigment thin films for photovoltaic cells. The only reference to particle size of the purified pigments is for vanadyl phthalocyanine, in which the particle size of the sublimed pigment, even after extensive milling (16 days), could not be reduced below 2 micrometers. A further reduction in particle size was obtained after acid pasting, a technique commonly used to generate pigment grade phthalocyanines (see R. Lambourne, "Paint and Surface Coatings", John Wiley and Sons (1987) p. 159).
H. Toyotama, in EPA 209403, describes a process for preparing dry ultrafine particles of organic compounds using a gas evaporation method. The ultrafine particles, having increased hydrophilicity, are taught to be dispersible in aqueous media. Particle sizes obtained are from 500 A to 4 micrometers. These particles are dispersed by use of ultrasound in order to provide mechanical energy to break up the aggregates, a practice known in the art. The resulting dispersions have improved stability towards flocculation.
Kimura and Bandow, Bull. Chem. Soc. Japan, 56, 3578 (1983) disclose the non-mechanical dispersing of fine metal particles. This reference describes a method for preparing colloidal metal dispersions in nonaqueous media also using a gas evaporation technique. General references by C. Hayashi on ultrafine metal particles and the gas evaporation technique can be found in Physics Today, December 1987, p. 44 and J. Vac. Sci. and Tech., A5, p. 1375 (1987).
Numerous references have appeared describing use of the gas evaporation technique to produce ultrafine metal powders, especially magnetic metal/metal oxide powders (often referred to as magnetic pigments). These refer to a dry process and do not involve contact with liquids. Yatsuya et al., Jpn. J. Appl. Phys., 13, 749 (1974), involves evaporation of metals onto a thin film of a hydrocarbon oil (VEROS technique) and is similar to Kimura (supra). Nakatani et al., J. Magn. Magn. Mater., 65, 261 (1987), describe a process in which surface active agents are used to stabilize a dispersion of a ferromagnetic metal (Fe, Co, or Ni) vaporized directly into a hydrocarbon oil to give a ferrofluid using a metal atom technique. The metal atom technique requires high vacuum (pressures less than 10.sup.-3 torr) such that discrete metal atoms are impinged onto the surface of a dispersing medium before the metal atoms have a chance to contact a second species in the gas phase. In this process, nucleation and particle growth occur in the dispersing medium, not in the gas phase. Thus, particle size is dependent on the dispersing medium and is not easily controlled. Additionally, U.S. Pat. No. 4,576,725 describes a process for making magnetic fluids which involves vaporization of a ferromagnetic metal, adiabatic expansion of the metal vapor and an inert gas through a cooling nozzle in order to condense the metal and form small metal particles, and impingement of the particles at high velocity onto the surface of a base liquid.
Other references for dispersing materials that are delivered to a dispersing medium by means of a gas stream include U.S. Pat. No. 1,509,824 which describes introduction of a molecularly dispersed material, generated either by vaporization or atomization, from a pressurized gas stream into a liquid medium such that condensation of the dispersed material occurs in the liquid. Therefore, particle growth occurs in the dispersing medium, not in the gas phase, as described above. Furthermore, the examples given are all materials in their elemental form and all of which have appreciable vapor pressures at room temperature.
UK patent 736,590 describes a process whereby a finely divided pigment is carried by a gas stream, wetted by a liquid miscible with the final dispersing medium while the finely divided pigment is suspended in the gas stream, and then the wetted pigment is mixed with the dispersing medium. This method requires a pulverizer to first subject the pigment to mechanical forces prior to its introduction into the gas stream. Therefore, it suffers from the shortcomings cited above. Particle sizes on the order of 1 micrometer are obtained.
U.S. Pat. No. 4,104,276 discloses the conversion of crude copper phthalocyanine into a pigmentary form by introducing milled copper phthalocyanine into an organic or aqueous medium together with a basic copper phthalocyanine of specified formula.
Pigmented water-absorbable plastic materials, including contact lenses, are disclosed in U.S. Pat. No. 4,638,025 to contain an organic binder, a pigment, a hydrophilic polymerizable plastic material, and a crosslinking agent. A binder such as cellulose acetate butyrate is required to keep the pigment in a dispersed form.
Contact lenses prepared from hydroxyethyl methacrylate containing copper phthalocyanine (commercially available and then milled) as a colorant are disclosed in U.S. Pat. No. 4,252,421.